Dr. Ammar Ganim Alhaaik, BVMS, MSc, PhD Scientific degree (Assist. Prof.), Department of Anatomy College of Veterinary Medicine, University of Mosul, Mosul, Iraq https://orcid.org/0000-0002-1473-7631 https://www.researchgate.net/profile/Ammar_Alhaaik4 Course name | Histology Part I | 2nd year 2019 Lymphatic Tissue 2 Spleen Located between the stomach, left kidney and diaphragm, the spleen is the largest lymphoid organ in the body, Functions • Removal of abnormal blood cells and particulate matter via phagocytosis • Storage of iron from recycled red blood cells • Initiation of the immune responses • Hematopoiesis in fetus and sometimes in adult The exterior surface of the spleen consists of a capsule containing collagen and elastic fibers and smooth muscle fibers covered with mesothelial cells. Capsule give rise trabecular connective tissue carry blood vessels toward parenchyma. The interior components are collectively called the "pulp". Upon gross examination of a slice of the spleen, the pulp has two very different appearances: red and white. The organ appears as a large expanse of red pulp dotted with white pulp. Histologically, red pulp is "red" due to the presence of large numbers of erythrocytes in blood vessels called sinusoids and white pulp is "white" due to lack of these sinuses and consequently fewer erythrocytes. The red pulp surrounds the white pulp while the latter looks like lymphatic nodules. Closer inspection of the white pulp indicates that there is a "central arteriole", sometimes called a central artery, close to the center of each area of white pulp. Histology P1 | Lymphatic tissue 2 | Dr. Ammar Alhaaik Page | 1 White pulp …… include lymph nodules and nodular or central artery + trabeculi Red pulp ………include sinusoids and splenic cords of lymphocytes Blood circulation within spleen: Splenic artery enters the hilum, divides into trabecular arteries, which are of variable sizes that follow the course of trabeculae. After leaving trabecula, these arteries will enter the parenchyma of spleen, and will be enveloped by a sheath of T-lymphocytes called peri arterial lymphatic sheath (PALS). These arteries are called central arteries or white pulp arteries. Although it is called central artery, it occupies an eccentric position in the white pulp, where it divides into many radial branches that supply the surrounding lymphoid tissue. After leaving white pulp, central artery subdivides to form straight penicillar arterioles with an outside diameter of 24µm. Near their termination, penicillar arterioles continues as arterial capillaries, which may be surrounded by Histology P1 | Lymphatic tissue 2 | Dr. Ammar Alhaaik Page | 2 macrophages, and thus called sheathed capillaries. When they leave the sheath, these vessels carry blood to the red pulp sinusoids. There are two theories to explain the flow of blood from capillaries to red pulp sinusoids: Closed circulation: where capillaries open directly into the sinusoids; the blood always remains inside the vessels. Open circulation: where blood pass through the spaces between the red pulp cord cells and then collected by sinusoids; the circulation will open into the parenchyma of the red pulp. Human circulation is of the open type. Tonsils Tonsils are large non-encapsulated (or partially encapsulated) masses of lymphoid tissue, that lie in the walls of the pharynx and nasopharynx The luminal surface of the tonsils are covered with epithelium while the base of tensile is encapsulated with C.T. The tonsils have many invaginations which form blind crypts. Below the epithelium, there are many lymphoid tissues (diffuse and nodular). ■ Palatine tonsils, located posteriorly on the soft palate, are covered by stratified squamous epithelium. The surface area of each is enlarged with 10-20 deep invaginations or tonsillar crypts in which the epithelial lining is densely infiltrated with lymphocytes and other leukocytes. The lymphoid tissue is filled diffusely with lymphocytes, with many secondary lymphoid nodules around the crypts. This tissue is underlain by dense connective tissue that acts as a partial capsule. 1- Covered by no keratinized stratified squamous epithelium 2- Have deep invaginations ( crypts( Histology P1 | Lymphatic tissue 2 | Dr. Ammar Alhaaik Page | 3 ■ The single pharyngeal tonsil is situated in the posterior wall of the nasopharynx, is covered by pseudostratified ciliated columnar epithelium, and has a thin underlying capsule. The mucosa with diffuse lymphoid tissue and lymphoid nodules is invaginated with shallow infoldings ( crypts). 1-Covered by Pseudostratified columnar epithelium 2-Have shallow invaginations (crypts) Histology P1 | Lymphatic tissue 2 | Dr. Ammar Alhaaik Page | 4 .